sulogin: agetty: use the plymouth local protocol instead the plymouth binary

sulogin: agetty: use the plymouth local protocol instead the plymouth binary

for stopping plymouthd.  That do not depend on the existence of
the plymouth binary if it e.g. becomes uninstalled or an other
service is providing plymouthd facilities.

[kzak@redhat.com: - fix compiler warnings [-Wpointer-sign]
                  - use sizeof() for write_all()
                  - cast to char* for read_all]

Signed-off-by: Werner Fink <werner@suse.de>
Signed-off-by: Karel Zak <kzak@redhat.com>